CityLink owner Transurban will reap more than $3 billion in extra toll revenue from the widening of CityLink and the Tullamarine Freeway, in a deal signed by the Andrews government despite “insufficient evidence” it will benefit the state, the auditor-general has found.

The $1.3 billion widening of CityLink and the Tullamarine Freeway between Melbourne Airport and CityLink’s tunnels was approved this year as a combined government-Transurban project.

The plan to widen one of Melbourne’s busiest freeways and boost its capacity by 30 per cent was pitched by CityLink owner Transurban in 2012 and ultimately approved by the Andrews government in April. It had also been approved by the former Napthine government, but was put on hold after Labor scrapped the East West Link.

Transurban will spend $850 million widening the toll road between Bulla Road and the tunnels, including Bolte Bridge, in exchange for a one-year extension to its concession deed with the state and higher tolls for heavy vehicles. It is expected the company will ultimately reap $3.2 billion in extra toll revenue from the deal.